This invention relates to an electrode or a lead which may be adapted to be connected to an electrical device and to contact a living organ to receive or conduct electrical signals therebetween. Notwithstanding its various uses, including applications in catheters and guide wires, this invention will be described as an endocardial pacing, and/or sensing lead for connecting a cardiac pacemaker and/or cardioverter/defibrillator pulse generator to cardiac tissue.
Body implantable cardiac pacemaker leads are quite well-known, their construction and function being described at length in medical journals during the past 25 years. The standard endocardial lead is of the type shown, for example, in U.S. Pat. No. 3,348,548, incorporated herein by reference, and comprises lengths of coiled wire conductors extending between a proximal pin(s) adapted to be connected to a pulse generator and a distal electrode(s) adapted to contact the endocardium of the heart. The lead is inserted and guided through a selected vein of the body until the distal end thereof is lodged in the apex of the right ventricle of the heart. A material, such as silicone rubber, or polyurethane, that is both electrical insulating and impervious to body fluids and tissue encases the coiled wire conductor either by a molding process or by insertion of the coiled wire conductor in a length of hollow silicone rubber, or polyurethane, tubing. In either case, a lumen extends down the center of the coiled wire conductor into which a stylet is advanced prior to insertion of the lead into the patient's vein to advance the lead through the patient's vein and to place the distal end of the lead bearing the electrode(s) at the desired position in the patient's heart.
Coiled wire conductor endocardial leads of the type disclosed in the aforementioned U.S. Pat. No. 3,348,548 employ a conductor that is circular in cross section. In more recent endocardial lead designs, multifilar conductors wound in parallel with a constant radius have been employed in order to use a smaller gauge conductor in order to, in turn, provide for a smaller diameter lead body which is both more flexible and more reliable than the single filar conductor described in the aforementioned U.S. Pat. No. 3,348,548.
Endocardial pacing leads have been categorized into passive or active fixation leads such as those shown, for example, by U.S. Pat. Nos. 3,902,501 and 4,106,512, respectively, each of which is herein incorporated by reference. The endocardial screw-in leads of the type shown in the aforementioned U.S. Pat. No. 4,106,512 employ the length of coiled wire conductor to transmit torque from the proximal connector end of the lead body to the distal fixation coil in order to advance that coil from the lead body and into heart tissue at a desired location within a chamber of the heart. In bipolar endocardial leads of the type described in the aforementioned U.S. Pat. No. 4,106,512, two conductor coils are coaxially arranged so that the inner coil may be rotated to rotate the fixation coil while the outer coil remains stationary. In this bipolar configuration, there are practical limits on the wire gauge which may be used to construct the inner coil inasmuch as it is required to transmit adequate torque to rotate the distal fixation mechanism.
Conductor coils from such wires with circular cross section have a fixed ratio between torque and bending stiffness. It is generally desirable to minimize the bending stiffness of the lead as a whole where that lead is provided with either a passive or active fixation mechanism to hold the distal electrode(s) in position within the heart. However, sufficient torque stiffness must be maintained to insure that the lead has adequate mechanical and handling characteristics. According to the present invention, the torque/bending stiffness ratio may be increased by using conductor coils wound with wires of non-circular cross section.